Heat exchanger tubes with longitudinal ribs



July 11, 1967 G. GOBEL HEAT EXCHANGER TUBES WITH LONGITUDINAL RIBS Filed March 23, 1965 4 Sheets-Sheet l INVENI'OP GER HHRD CabBEL July 11, 1967 4 Sheets-Sheet 2 Filed March 23, 1965 INVENI'OIP GERHHRD 6635!.

July 11, 1967 G. GGBEL. 3,330,336

HEAT EXCHANGER TUBES WITH LONGITUDINAL RIBS Filed March 25, 1965 4 Sheets-Sheet C5 FIG. /0a

GERHHRD GijBEL July 11, 1967 G. GbBEL HEAT EXCHANGER TUBES WITH LONGITUDINAL RIBS 4 Sheets-Sheet 4 Filed March 23, 1965 INVENIOP GERHHRD GCSBEL United States Patent i G4 14 Claims. (Cl. 165-160) The invention relates to heat exchangers composed of tubes or pipes, which are provided with flat needles or short ribs pointing in the longitudinal direction of the tubes. The media flow outside the tubes and essentially in their direction. These tubular heat exchangers are employed particularly in chemical process devices and refineries, especially as preheaters for viscous oils, but also are used in other fields of endeavor. It should be pointed out that not only viscous liquids but liquids of normal flow and gases also can be used in the heat exchangers according to the invention.

Heat exchangers using nests or bundles of tubes of various construction are known. These usually are provided With bafiles in order to increase the flow velocities and the heat exchange. At moderate heat exchange values, the pressure loss and the expenditures of weight, volume and cost are comparatively high.

Other heat exchangers are known which have longitudinally ribbed tubes, wherein the thin ribs are interrupted at relatively great distances. At small pressure losses, only very low heat transfer values are obtained with this construction, and the costs are approximately as high as those using smooth tube heat exchangers. There also are needle rib tubes on whose circumferences circular needles are disposed which, in part, are bent in tangential direction in order to cause a pitch of the stream about the disposed tubes. Finally, there is a construction wherein the traversing ribs are disposed on the connecting line with neighboring tubes, wherein a pitch is created by bafile plates installed in a triangular space defined by the tubes and the longitudinal ribs. This pitch substantially causes tangential flow relative to the tube and rib surface.

In the present invention, ribbed tube heat exchangers have tubes which are arranged in triangular or quadrangular disposition, whereby the tubes, in quadrangular arrangement, are provided with four, and in triangular arrangement with 6, single or multiple rows of short needles or ribs in the direction of the flow, and whereby all needles or ribs, respectively, of the neighboring tubes form rib stars whose centers are situated essentially within the center of gravity of the triangles and squares, respectively, the majority of the ribs pointing to the center of the star.

Furthermore, the flat needles or short ribs, arranged in consecutive rows, are at an angle of incidence relative to the tube axis, so that a pitch is effected in the flow of the outer medium about the center of the star, whereby the pitched flow crosses the individual rib rows. Within a single rib star, all ribs are inclined in the same direction relative to the star center. Neighboring rib stars opportunely have opposite direction of pitch, and these ribs, correspondingly have opposite angles of incidence.

The heat exchangers as described above have the following advantages over those hitherto known:

1) High specific heat transfer relative to the outer surface, even at lowest flow velocities and at slight pressure drop, not only at the ribs but also at the tube surfaces.

(2) Thermically most favorable and entirely uniform filling of the interior cross section of the exchanger with tubes and rib surfaces causing uniform heat distribu- 3,339,335 Patented July 11, 1967 tion and optimum utilization of the heating surfaces. No dead space, especially in the areas of high tempera, tures, i.e., on the tube walls, hence no danger of encrustation due to local overheating.

(3) Essentially a laminar flow is attained without vortification. No current paths having higher temperature are mixed with those having lower temperature, so that the highest effective temperature diiference is utilized.

(4) Even with narrow tube divisions the largest possible rib lengths and, hence, rib heating surfaces are obtained, and sufiiciently large cleaning channels remain, accessible from the front.

(5) Considerable reduction of the volume of the exchanger and its weight with corresponding lowering of costs.

(6) The considerable reduction of the weight and of the surface enable utilization of an effective surface protection or of corrosion-resistant material while remaining within economical range.

The invention now will be further explained with reference to the accompanying drawings. However, it should be understood that this is given merely by way of explanation, and not of limitation, and that numerous changes may be made in the details without departing from the spirit and the scope of the invention as hereinafter claimed.

In the drawings,

FIG. 1 is a cross section of a nest or bundle of tubes in a heat exchanger system.

FIG. 2 is a projection of the periphery of a tube with six rows of ribs.

FIG. 3 is a cross section of an embodiment showing a quadrangular arrangement.

FIG. 4 is a diagram showing the effective temperature difference over a given rib length.

FIG. 5 is a plan view of a row of ribs according to FIG. 3.

FIGS. 6 and 7 show two manners of forming the ribs.

FIG. 8 is a cross section through a heat exchanger having three ribbed tubes in a jacket.

FIG. 9 is a plan view of, and

FIG. 9a a cross section through, a butterfly rib band in several stages of fabrication.

FIG. 10 is a plan view of a needle band in various fabrication stages.

FIG. 10a is a cross section of a portion of FIG. 10.

FIG. 11 is a plan view of a U-strap or -loop in various fabrication stages.

FIGS. 12 and 13 are longitudinal sections through two embodiments of heat exchangers provided with U-tubes.

FIG. 14 is a schematic in section of a heat exchanger equipped with ribbed tubes.

FIG. 15 is a section through a ribbed tube with an injector tube disposed therein.

FIG. 16 is a section through a tube provided with inner ribs.

FIG. 17 is a section, and

FIG. 18 is a plan view of, a stage of a tube manufacture.

FIG. 19 is a longitudinal section through, and

FIG. 20 is a cross section of, a ribbed tube according to a special embodiment of the invention.

Referring now to these drawings,

FIG. 1 shows, in cross section, a triangular arrangement of a plurality of tubes 1 for heat exchangers according to the invention. On the periphery of these tubes 1, siX rows of ribs 2 are disposed which form stars of given pitch with the ribs of neighboring tubes, whereby three ribs of different tubes belong to each star in the arrangement shown.

In FIG. 2 a projection of the periphery of a tube is shown having six rib rows. Neighboring rows of ribs 2 each have opposing angles of incidence 6 relative to each other. By means of angle 6 of the short ribs 2 which point in the direction of the current flow, the flow of the medi um is diverted in direction 3, and each rib obtains an independent current path. The slight depth of the ribs in the direction of the current flow is shown as 8, and

9 denotes the median clearance. The depth 8 preferably is no more than 30 mm. for liquids and no more than 60 mm. for gases.

FIG. 3 shows, in cross section, an embodiment using a quadrangular, or square, arrangement. In this instance, tubes 1a, 1b, and 10, respectively, are provided with four rib rows 2a, 2b, and 20. One rib of each of the four tubes, disposed on the corners of a square, is directed toward the common star center in the center of the square (its center of gravity). All ribs of this star have angles of incidence 6 which are of like direction relative to the star center, so that the medium, flowing in axial direction of the tubes, is imparted a superimposed pitch to the left or to the right, intersecting the rib rows. The several current paths 4a, 4b, 4c of the medium are shown by lines and arrows. The outermost, longest current path 40, e.g., consecutively intersects the four tubes. This current path 40, corresponds to the largest volume and weight, respectively, of the medium to be heated, whereas current path 4a is comparatively short. A considerably smaller weight of the medium to be heated, therefore, belongs to path 4a, at approximately equally large heating surface of the ribs. This means that the heating surface per weight unit to be heated increases toward the star center 5. In this manner, the temperature decrease in the ribs substantially is compensated.

FIG. 4 is a diagram showing the eifective temperature difference 19 over rib length 10, starting from tube 1 to star center 5. Line 13 denotes the corresponding current path volume or current path weight, respectively. The eifective temperature difference, decreasing toward star center 5, hence, is compensated by a decreasing weight to be heated or a larger heating surface density, respectively. The rib opportunely is calculated, regarding length 10, rib thickness, rib material and u-value, so that the temperature difference 19a at the rib head is approximately 25 percent of the difference at the tube.

FIG. 5 is a plan view of a rib row according to FIG. 3.. The individual ribs 2a have a profile favoring the flow and also are arranged inclined by the angle of incidence 6 relative to the axial direction of the tubes. These ribs may be cut from profiled bands and applied to tube 1a, according to FIG. 3, by butt welding, pressure welding, or similar process.

However, the entire ribbed tube 1b can consist of an extrusion piece having tube and ribs 2b in one piece. Then, according to FIG. 6, the originally continuous long ribs are slit and their ends 20 bent and thus are converted into short ribs having a diverting effect.

As a further alternative, the ribbed tube (FIG. 3) can be formed by welding rib bands 2c (FIG. 7) thereto. The bands are subdivided by slitting and bending into short ribs. The weld is shown in FIG. 3 as 24.

FIG. 8 shows the cross section through a heat exchanger according to the invention having three rib tubes in a jacket 12. The tubes 1 are arranged triangularly, and each tube, in this embodiment, is provided with six dual rib sets 2c. Each of the three tubes provides a rib pair 11'to the central rib star 5. The rings 4 with arrows show the different pitch of the currents. As can be seen from this FIG. 8, the incomplete rib stars, i.e., those consisting of merely two or one rib pair 11, also cause a current pitch and encompass in this manner the zones outside the actual rib tubes and their ribs. However, it also is feasible to fill out the inner cross sections with displacement sheets or bafiles 18.

According to the prevailing conditions and at a maximum length of approximately 4 111. (four meters) a rib tube is capable of transferring only a definite quantity of heat to which a definite volume of the exchange media ribs (in this embodiment double the amount) can be pro- I vided.

The geometrical fact that in a triangular arrangement solely six single ribs or six groups of ribs with two or more ribs, respectively,'can be installed on the periphery; that, moreover, the rib length 10 is limited; and the largest circles of pitch 4c must intersect, results in the requirement of a definite dimensioning for multiple tube sys tems. Therefore, the tube diameters are not larger than mm. and the division for liquids not more than mm. and for gases not more than mm. A hexagon 14 is shown for each ribbed tube. At given points of the corners 5, spacers 15 or the like can be installed in order to assure exact position of the tubes. Within the realm of the spacers opportunely a strap or clamp surrounds the entire nest of tubes. As a special embodiment, it also is feasible to have a single rib tube in a jacket 12 whereby a gap 16 remains above the single rib body, assuring the possibility of pitching the current or flow.

Rinsing or flushing means can be inserted in the compartively large cleansing channels 17.

A six-rib group system lends itself to simultaneous tripleor sextuple welding using the customary tn'phase current while avoiding the drawback of nonuniform phase load.

In the embodiment shown in FIG. 8, the tubes are provided with so-called butterfly rib bands 2c. These are installed alternately in opposed directions. As alternatives, several other possible embodiments of the ribs are shown on tubes 1d and 1e. 2d is a butterfly needle band, and 2e and 2 are U-loops or -straps. For the latter, the correlating rib pairs 11 of one ribbed tube are combined to a single unit.

FIG. 9 is a plan view of the butterfly rib band 20 in its several stages of fabrication. An endless metal sheet strip 21 first is subdivided into short ribs by slitting on both sides 22. Thereafter, the ribs obtain a flow-favoring profile by upsetting the rectangular :slit edges 23, and, finally, in the third process step, both the ribs are bent upwardly in the shape of a V and provided with the angle of incidence 6 (see FIG. 9a). It is opportune to interpose a calibrating step in order to obtain a surface 24 exactly fitting tube 1 (see FIG; 3). This assures high-quality electric spotor seam welding without spattering and good heat contact. In this embodiment, both shanks of the V-shaped band have opposite angles of incidence.

FIG. 9a is a section through a rib row with upset edges 23.

FIG. 10 illustrates the several different fabrication steps of a needle band 20. First, an endless metal sheet band is slit on both sides in distances of one to three times the thickness of the sheet 22 and then the strips 25 thus formed are bent differently into V-shape so that twicestaggered rows of needles are obtained (see FIG. 10a) providing sufficiently large cross sectional areas 26 transverse to the axis of the tube. This band also is combined with the tube by spot-, spotand stitch-, seam-Welding, or by soldering, or in a similar manner.

The U-straps or -loops 2e and 2] as shown in FIG. 8 have angles of incidence in like direction. The U-straps may be single units or else can be combined to bands by means of a clamp 27 disposed at the heads. The U- straps are joined at their shank ends to the tube at 28. In the embodiment 22, these shank ends may be bent to form a straight line to enable automatic flash welding. From a caloric point of view, the embodiment having a staggering foot line is better; however, this permits solely butt press welding or soldering or a similar process. The divisions of the shank ends on the periphery of the tubes are alike and, in this instance, are of said periphery.

U-loops thus applied have the advantage over the butterfiy version of smaller temperature decrease in the transition zone from tube to rib.

FIG. 11 illustrates the several process steps of fabricating of U-strap or -loop having a staggered foot line 2 An endless metal sheet band first is perforated in the center, then is provided with sloping slits 22a, and the shank ends are given fitted cuts 28a. The shanks then are bent to U-shape. At the heads an uninterrupted strap 27 remains.

FIG. 12 shows a longitudinal section through a heat exchanger having U-tubes. Therein solely the upper shanks 30 of the U-tubes are provided with ribs, while the lower smooth shanks 31 have a very slight distance from each other. The countercurrent principle, advantageous from a heat transfer point of view, thus largely is utilized, especially if liquids traverse the inside of the tubes.

Even more favorable is the embodiment according to FIG. 13. Therein a straight bundle or nest of rib tubes 30 is connected to a single reflux tube 33 by means of thin connecting tubes 32. The portion of the heating surface of the reflux tube 33, disposed in the continuous current, thereby becomes negligible. The thin tubes 32 also compensate for heat expansion and are installed in such a manner that accessibility to cleansing channels 17 from the front remains. In FIGS. 12 and 13, 34 is, e.g., an oil inlet, 35 an oil outlet, 36 a steam inlet, and 37 the condensate outlet.

FIG. 14 is a schematic in section of a heat exchanger having ribbed tubes 1d, Which are closed on one end and are fastened solely at their open ends to tube sheet 38. The heating medium (water, steam) is conducted from the distributor chamber 39 by way of injector tubes 40 into the ribbed tubes 1d and flows back in opposite direction. These ribbed tubes 1d, in this embodiment, are loose construction elements, gasketed with O-rings 53 so that they may readily be taken apart for cleaning purposes. The ribbed tubes are held in place by the divided ring 54.

The injection tubes 40 herein also are loose elements, the tube sheet 38 is divided and has a neutral central zone 41 which prevents mutual contact of the two media in case of leaks. It also is feasible to put electric heating elements into the ribbed tubes.

Three different types of ribs are disposed on tube 42, as follows:

(1) At the inlet, in the area of high pressure drop with viscous media, thick, short ribs 43, under certain conditions in lesser amounts relative to the circumference. These ribs must have as high as possible a surface temperature and small surface area at large clearance (between the individual ribs) to reduce the pressure drop.

(2) In the main portion having well-defined longitudinal flow, normal ribs 44 are present.

(3) In the outlet area considerable cross current occurs for which the inside diameters of the normal rib profile are too small. Here, needle ribs 45 or the needle rib band 2d, respectively, are employed.

FIG. 15 is a section through a ribbed tube 1 with an injection tube 40 disposed therein which carries a ribbed jacket 46. The latter, by means of the disposition of ribs 2g, similar to those shown in FIG. 2, provides for differently pitched currents especially in the case of liquids. These currents always contact the inner wall only at short stretches 47. This considerably increases the heat transfer effect which is highly important on account of the intense heat transmission to the outside through the pitched n'bs. Jacket 46 opportunely is made of a material having low heat conductivity in order to reduce the undesirable heat transfer from the inside of the injector tube to the outer periphery of the ring.

FIG. 16 is a section through a tube having inside ribs 2h which also have a disposition similar to FIG. 2. The arrows show a superimposed pitch of the current which considerably increases the heat transfer to the tube wall.

This tube is manufactured, as shown in FIG. 17 in section and in FIG. 18 in plan view, from a metal sheet band into which the ribs 2h previously had been rolled and also, if desired or required, outer ribs 21'. The metal band then is bent to a tube and welded in the customary manner. These tubes with internal ribs primarily are used for liquids.

FIG. 19 is a longitudinal section, and FIG. 20 a cross section, through a ribbed tube in vertical axial disposition wherein the current flow is effected essential by natural uplift or thermal convection. This embodiment is suitable, among other purposes, for the heating of tanks. In this single-tube embodiment more than six rib groups may be disposed on the periphery. The arrows denote that, beside the pitched fiow 4, a suction current 48 of unaffected medium also occurs along the entire length of the tube which promotes a considerable increase in the heat transfer. The heat exchange carries into the areas 49 which considerably exceed the areas of the ribs themselves. Even at the low velocities attained by the uplift current substantially equal heat transfer values can be attained as with forced currents, due to extremely small rib depths 8. This tank heating element also opportunely is closed solely on one side, whereby the lower, open, portion is connected to a single steam distributor pipe 50. An injector tube 40a protrudes into pipe 54}. Injector tube 40a is provided with an opening 51 directed in opposition to the stream cur-rent 52 so that steam flows into the ribbed tube by means of the pressure head. The condensate is removed from the wake of the injector tube 40a. The steam distributor pipe 50, in this instance, also serves for the division of the condensate.

In accordance with the relationship Fro/ m) the depth 8 of the ribs (1)) (see FIG. 2) in dependence of the length of the tubes, measured in the direction of the current, and the flow velocity u, is determined in such a manner that the ocand the k-value retain approximately equal optimum size. b varies proportionally with the total length.

The same result is obtained With the following calculations according to which the number of consecutive ribs of one row remains approximately equal at different length, for instance, -300 ribs in heat exhangers for liquids, and 200500 ribs in heat exchangers for gases. Moreover, it is opportune to increase the pitch in small size exchangers, i.e., to increase the angle of incidence 6, which produces an increased superimposed velocity.

In the area of laminar flow of highly viscous liquids, e.-g., heavy oil, and at low flow velocities, e.g., less than 0.5 m./sec. solely the skin friction is worth consideration for the pressure drop Ap. Essentially, the equation Ap=f(uLv/s is valid, wherein u is the flow velocity L is the total length s is the distance between ribs, i.e. the width of the slits v is the viscosity.

High u-values cause smaller total lenth L and larger widths s because of the decreased heating surface areas, and therefore smaller pressure losses. This contradicts the generally valid rules.

Therefore, the slit width s 9 advantageously is larger than 10 mm. The repeated slitting and the large quantity of short ribs and, thus, the large number of starting runs is practically of no significance at all due to the low velocities.

I claim as my invention:

1. In a heat exchanger having an essentially cylindrical, closed shell with inlet and outlet means disposed at the extreme ends of said shell, a plurality of tubes disposed therein, and inner and outer media flowing in and about said tubes, respectively, wherein at least one row of ribs is disposed on the outer periphery of said tubes, said tubes being disposed in geometrical relationship relative to each other, the spaces the-rebetween forming a definite pattern, one row of ribs on each tube pointing to the center of said pattern and forming stars of given pitch with ribs of adjoining tubes; the improvements which comprise said ribs being straight, blunt-ended, of equal length, and equidistantly disposed on said tubes; all ribs being inclined at like oblique angles of incidence relative to said 'tubes; ribs on one tube being inclined in like direction; neighboring tubes carrying ribs of opposite angles of in-. cidence, thereby causing a pitch in the flow of the outer medium about the center of said stars and thus a corkscrew pitch about and tangential to a plurality of said tubes.

2. A heat exchanger for liquids and gases comprising a substantially cylindrical closed shell; a plurality of tubes within said shell, disposed in geometrical relationship relative to each other; inner and outer media flowing in a said tubes, pointing toward each other thus forming pat and about said tubes, respectively; inlet and outlet means a for said media at the extreme ends of said shell; at least one row of short, straight, blunt-ended ribs of like size equidistantly disposed on the outer periphery of said tubes, inclined at like oblique angles of incidence relative to said tubes, pointing toward each other and forming a definite geometrical pattern with ribs of adjoining tubes; neighboring tubes carrying ribs of opposing angles of incidence, thereby causing a flow of the media parallel to the axis of said tubes and a corkscrew pitch in the flow of the outer medium about the center of said pattern, crossing individual rows of ribs tangentially to' a plurality of said tubes.

.o o to three times their thickness and are bent to form a staggered row on said tube. V

11. The heat exchanger as defined in claim 2, wherein said ribs are in the form of U-st-raps, each shank being connected to said tube. V

12. The heat exchanger as defined in claim 2, wherein ribs also are provided on the inside of said tubes.

13. A heat exchanger for liquids and gases comprising a substantially cylindrical closed shell; a plurality of tubes within said shell in quadrangular disposition relative to each other; inner and outer media flowing in and about said tubes, respectively; inlet and outlet means for said media at the extreme ends of said shell; at least one row of short, straight, blunt-ended ribs of like size equidistantly disposed on the outer periphery of said tubes, inclined at like oblique angles of incidence relative to terns of squares with ribs of adjoining tubes; neighboring tubes carrying ribs of opposing angles of incidence, thereby causing a fiow of the media parallel to theraxis of said tubes and a corkscrew pitch in the flow of the outer medium about the center of said square, crossing individual rows of ribs tangentially to a plurality of tubes.

14. A heat exchanger for liquids and gases comprising a substantially cylindrical closedshell; a plurality of tubes within said shell, disposed ingeometiical relationship toward each other; inner and outer media flowing in and about said tubes, respectively, inlet and outlet means for said media atthe extreme ends of said shell; at least one row of short, straight, blunt-ended, V-shaped ribs of like size equidistan-tly disposed on the outer periphery of said tubes, inclined at like oblique angles of incidence relative to said tubes, pointing toward each other and forming a boring tubes; neighboring pairs of ribs contacting each other so as to form a plurality of inverted V-shapes on the periphery of said tubes.

4. The heat exchanger as defined in claim 2, wherein said ribs are flat.

5. The heat exchanger as defined in claim 2, wherein said ribs are needle-shaped.

6. The heat exchanger as defined in claim 5, wherein short and thick ribs are disposed on said tube near the inlet end for said medium in lesser quantity than near the opposite end; and said needle ribs being disposed on said opposite end. V

7. The heat exchanger as defined in claim 2, wherein the depth of said ribs, measured in the flow direction, is no more than 30 mm. for liquids and no more than 60 mm. for gases.

8. The heat exchanger as defined in claim 8, wherein the depth of the ribs in proportion to the length of the tube and the number of consecutive ribs per tube is determined by the equation wherein b is the depth and u is the flow velocity.

9. The heat exchanger as defined in claim 2, wherein the ribes are V-shaped, bent at an angle and interconnected by a continuous center strip fastened to said tube.

10. The heat exchanger as defined in claim 9, wherein said ribs are at a distance from each other equalling one definite geometrical pattern with ribs of adjoining tubes; neighboring tubes carrying ribs of like angles of incidence, thereby causing a flow of the media parallel to the axis of said tubes and a corkscrew pitch in the flow of the outer medium about the center of said pattern, crossing individual rows of ribs tangentially to a plurality of said tubes.

References Cited UNITED STATES PATENTS 1,770,320 7/1930 Morterud -147 X 1,920,800 8/1933 McCausland 165-160 1,935,412 11/1933 Price 165160 X 2,322,284 6/1943 Dewald 165 142 2,444,908 7/1948 Bailey et a1. 165-172 X 2,905,447 9/1959 Huet 165179 X 3,020,027 2/1962 Dumpleton 165-18 1 3,118,495 1/1964 Hagby 165142 X FOREIGN PATENTS 578,013 6/1959 Canada. 74,384. 11/ 1960 France. 688,641 5/ 1930 France. 835,612 3/1952 Germany. 1,065,864 9/1959 Germanyr 366,262 2/1932 Great Britain. 371,347 4/1932 Great Britain. 619,672 3/ 1949 Great Britain. 670,598 4/ 1952 Great Britain.

ROBERT A. OLEARY, Primary Examiner.

' M. A. ANTONAKAS, Assistant Examiner. 

2. A HEAT EXCHANGER FOR LIQUIDS AND GASES COMPRISING A SUBSTANTIALLY CYLINDRICAL CLOSED SHELL; A PLURALITY OF TUBES WITHIN SAID SHELL, DISPOSED IN GEOMETRICAL RELATIONSHIP RELATIVE TO EACH OTHER; INNER AND OUTER MEDIA FLOWING IN AND ABOUT SAID TUBES, RESPECTIVELY; INLET AND OUTLET MEANS FOR SAID MEDIA AT THE EXTREME ENDS OF SAID SHELL; AT LEAST ONE ROW OF SHORT, STRAIGHT, BLUNT-ENDED RIBS OF LIKE SIZE EQUIDISTANTLAY DISPOSED ON THE OUTER PERIPHERY OF SAID TUBES, INCLINED AT LIKE OBLIQUE ANGLES OF INCIDENCE RELATIVE TO SAID TUBES, POINTING TOWARD WACH OTHER AND FORMING A DEFINE GEOMETRICAL PATTERN WITH RIBS OF ADJOINING TUBES; NEIGHBORING TUBES CARRYING RIBS OF OPPOSING ANGLES OF INCIDENCE, THEREBY CAUSING A FLOW OF THE MEDIA PARALLEL TO THE AXIS OF SAID TUBES AND A CORKSCREW PITCH IN THE FLOW OF THE OUTER MEDIUM ABOUT THE CENTER OF SAID PATTERN, CROSSING INDIVIDUAL ROWS OF RIBS TANGENTIALLY TO A PLURALITY OF SAID TUBES. 